1. Field of the Invention
The present invention relates to a brake control device in an antilock brake system.
2. Description of the Prior Art
The present invention is related to an antilock brake system, and particularly to a brake control device in an antilock brake system.
Prior Art:
An antilock brake system is a brake control system which automatically reduces the brake cylinder fluid pressure (hereinafter "brake fluid pressure") to lower the brake pressure and cancel wheel locking when the brakes are applied and the wheels lock and begin to slip on the road surface; when this antilock control is applied it overrides the operation of the brakes by the driver.
In FIG. 1 is shown a conventional brake pressure control system. When the brake pedal is pressed, the fluid pressure inside the brake cylinder 2 rises, and the brakes 4 are applied by means of the intake solenoid valve 3. If the wheels lock because the brakes are excessively applied, the antilock control system functions: the intake solenoid valve 3 is displaced to the pressure rise buffer position and the exhaust solenoid valve 6 is displaced to the conductivity position for the calculated time period .DELTA.T. The fluid pressure applied to the brakes 4 is thus lowered, the brake 4 pressure decreases, and the wheels can be released from the locked state.
Problem to be Solved by the Invention:
In general, brake fluid pressure characteristics vary non-linearly as shown in FIG. 2(a). As a result, the higher the brake fluid pressure is when pressure reduction is first applied, the wider pressure reduction band .DELTA.P becomes for the same time period .DELTA.T. Thus, in order to obtain a given pressure reduction band, the time period .DELTA.T must vary according to the brake fluid pressure.
However, since conventional devices do not compensate the pressure reduction time according to the actual or estimated brake fluid pressure, antilock control begins at a low brake fluid pressure on frozen roads, snow-covered roads, and other road surfaces with a low coefficient of friction .mu. ("low .mu. surfaces"); this results in excessive tire slipping, as well as reduced steering control on front wheel drive vehicles, and reduced vehicle stability on rear wheel drive vehicles due to insufficient reduction of the brake fluid pressure. Furthermore, on asphalt and other road surfaces with a high coefficient of friction .mu. ("high .mu. surfaces"), antilock control begins at a high brake fluid pressure, resulting in excessive brake fluid pressure reduction which may cause vehicle deceleration to fluctuate and the required braking distance to increase.